Switch modules with power semiconductor devices are used in converters for the conversion of direct current (DC) to alternating current (AC) and vice versa, and for many other applications. A switch module represents a controllable part, whose state can be changed by a control signal between a conducting (or on) state and a non-conducting (or off) state. For high performance applications, for example, high voltage-direct current (HVDC) applications, in many multi-level converter topologies, often several switch modules are connected to each other in series. Redundancy requirements, according to which the function of a device must be ensured in case of a failure of one or more switch modules, can require the application of two or more switch modules connected in series.
It is important that a faulty switch module does not influence the functioning of the non-faulty switch modules or the power electronic module as a whole, for instance, a converter. In this respect, it is desirable that in a series of switch modules a faulty switch module can be set in a short circuit failure mode, in which it continuously conducts so that the operation of the power electronic module can be continued with other functionally capable switch modules. In many circuit topologies, for instance, H-bridges, half-bridges etc., is useful if a switch module is moved to a failure mode in a conducting state so that both the alternating voltage connections of an H-bridge can be continuously conductively connected with each other.
A switch module can include a number of power semiconductor devices or switching elements, e.g., insulated-gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs) or other similar devices, which are connected in parallel to each other and switched simultaneously. Thereby the current load can be distributed between several parallel circuits, which enables operation with high current and output. A drive unit is provided in order to feed the switching elements of a switch module with a suitable common control signal on the basis of a control signal from a superior control. The switching elements can be accommodated within a common housing, e.g., a pressure contact housing (so-called press-pack housing). The housing can contain other components like symmetrical resistances in the control circuit in order to achieve symmetry, and to achieve maximum possible simultaneous and equal switching of all the switching elements in the case of external control. The drive unit is normally arranged outside the housing and is connected to one or more terminals that extend out of the switch module housing.
If one of the parallel connected switching elements, for example, an IGBT, suffers a short circuit between its collector electrode and its emitter electrode, and as a result becomes faulty and is destroyed, this generally leads to a short circuit between the gate electrode of the switching element and its emitter electrode. The gate-emitter short circuit of the faulty switching element, due to parallel switching, also shorts the gate electrodes of other, functionally capable switching elements to the emitter electrode and prevents the gate electrodes from maintaining a sufficient voltage so as to remain in a conducting state or to move to a conducting state. Consequently, a control of the functionally capable switching elements is impossible through the gate electrodes.
A failure of a switching element, hereinafter referred to also as a fault, is understood to mean a short circuit of the gate electrode to another electrode, especially the emitter electrode.
Under these conditions, the faulty switching element must conduct the complete load current and manage it. The current load of the faulty switching element with nominal current of the switch module or possible overcurrent, and the associated excessive heating, can lead to destruction of the switch module and also damage the drive unit and other system components. Especially, electric arcs can emerge, which can lead to outage of other components, an explosion of components or fire. This should be prevented. Furthermore, a low Ohm (or low impedance) current path should be installed through the faulty switch module in order to ensure further functional capability of a power electronic module.
In order to avoid a mechanical destruction of the switch module through explosion, the switching elements so far have been protected against explosion by explosion-proof metal housings, which is difficult and expensive. Complex ultra-fast explosive driven mechanical bypass switches are also used for converters. Still further, switch modules are developed which present a stable short circuit failure mode (SCFM) in case of a fault. The SCFM-capability is achieved by placing a metal plate on top of the switching elements of a power electronic module (see e.g., EP 0989611). A failure melts the metal plate and the switching element and forms a conductive metal-silicon alloy, which enables a low Ohm (or low impedance) current path through the switch module. Due to material oxidation, if the electrical resistance of the damaged switching element increases, another electrically parallel connected switching element will be damaged and it melts and forms the next low Ohm (or low impedance) current path. These types of SCFM transitions are continued till all the switching elements are used up. In spite of the high complexity, a continuous and stable short circuit failure mode cannot be ensured.
WO 2006/104430 A1 describes a power converter valve and a control process which uses several parallel connected semiconductor switching elements, whereby to each switching element a separate drive unit is assigned. A fault in one of the switching elements is recorded through a current measurement, and the other, fault-free switching elements are further powered specifically through their associated drive units in order to create a conducting state of the converter valve. The short circuit protection is gained with high circuit and control expenditure.
WO 2013/139373 A1 describes a switch module with a first connection, a second connection, a gate, several switching elements, which are connected to each other in parallel between the first connection, the second connection and the gate, fuses, which are provided between each individual electrodes of several switching elements and the gate, and a bypass switch, which is arranged between the gate and the second connection. If a short circuit fault is identified in a switching element, the bypass switch is triggered and closed in order to branch out a part of the load current flowing through the short circuited switching element and to conduct through the associated fuse, a bypass resistance and bypass switch to the second connection for the switching element in order to melt the fuse. As soon as the fuse melts, the control line to the faulty switching element is broken so that the rest of the switch module remains functional. However, this solution requires an additional active switch within the switch module and an active logic for controlling this additional active switch in case of fault.
Based on this, it is the objective of the invention to create a switch module with several parallel connected switching elements (e.g., power semiconductor devices), which is in a position to achieve at least a limited controllability of all or part of the remaining non-faulty or functionally capable switching elements after a fault in a switch module or in an individual switching element in order to enable a continuously conducting state of the switch module, so as to reduce the danger of an explosion and to create a stable low Ohm (or low impedance) current path through the faulty switch module. This mostly involves simple means and with simple structure, with less expenditure and lower costs with the manufacturing and operation.
Another objective of the present invention is to create a power electronic module with a switch module, which is particularly suitable for high performance application.